1. Field of the Invention
The present invention relates to a method of controlling roll eccentricity of a rolling mill and an apparatus for performing the same.
2. Description of the Prior Art
In the thickness control of a rolling mill, it is difficult theoretically to remove a rolling load variation due to an eccentricity of backup rolls thereof by using the conventional gaugemeter method or the X ray thickness meter having feedback system. In order to overcome the difficulty, it has been proposed to estimate a rolling load variation due to the eccentricity of the backup rolls correspondingly to a rotation angle of the rolls and to preliminarily control the rolling mill on the basis of the estimated value. The latter method is disclosed, for example, in Japanese Patent Publication No. 53-16386 and Japanese Patent Application Laid-open No. 52-65158 both of which are assigned to the assignee of the present application. The methods disclosed in the Japanese Patent Publication and the Japanese Patent Application Laid-open will now be briefly described by referring them as first control method and second control method, respectively.
The first control method;
The first control method is practicized by using an apparatus shown in FIG. 1. That is, in a first roll rotation period, a pulse signal obtained by a pulse generator 12 directly connected to an upper backup roll 1 and an output signal of a load cell 9 which functions to detect a rolling load variation are supplied to a roll eccentricity control device 11. In the roll eccentricity control device 11, the pulse signal is converted into a rotation angle .theta..sub.T of the backup roll 1 and a rolling load variation value V.sub.1 (.theta..sub.T) which is due to eccentricity of the upper and a lower backup rolls 1 and 4 is obtained from the output signal of the load cell 9 as a function of the rotation angle .theta..sub.T, with using a process to be described.
In a second roll rotation period, the rolling load variation value V.sub.1 (.theta..sub.T) obtained in the first period is used as a roll eccentricity compensation signal for the rolling mill. That is, the rolling load variation value V.sub.1 (.theta..sub.T) which is an output of the control device 11 is supplied to a rolling force control device 10 of oil pressure type to control a rolling force cylinder 6 through a servo valve 8 to thereby compensate for the rolling load variation due to the eccentricity of the rolls.
A rolling load variation value V.sub.2 (.theta..sub.T) is also obtained in the second period as in the first period. A sum of the rolling load variation values V.sub.1 (.theta..sub.T) and V.sub.2 (.theta..sub.T) is used in a third roll rotation period as a roll eccentricity compensation signal for the rolling mill for that period.
Therefore, in n-th roll rotation period, a roll eccentricity compensation signal to be used to control the rolling mill becomes V.sub.1 (.theta..sub.T)+V.sub.2 (.theta..sub.T)+. . . +V.sub.n-2 (.theta..sub.T)+V.sub.n-1 (.theta..sub.T).
In FIG. 1, reference numerals 2 and 3 show the work rolls, 7 shows a deviation meter for detecting a deviation of the rolling force cylinder 6 and 5 shows a sheet material to be rolled.
FIG. 2 shows a method of obtaining from the output signal of the load cell 9 the rolling load variation values V.sub.1 (.theta..sub.T), V.sub.2 (.theta..sub.T) . . . which are due to the roll eccentricity. The output signal of the load cell 9 has a waveform including a rolling load variation component 2B due to the thickness variation of the sheet material 5 to be rolled superimposed with a rolling load variation component 2C due to the roll eccentricity which is shown in a lower part of the same figure. Therefore, in order to obtain the rolling load variation component 2C, it is necessary to subtract the rolling load variation component 2B from the output signal 2A of the load cell 9. The subtraction may be performed as follow.
Since the rolling load variation 2B due to the thickness variation of the sheet material 5 changes slowly with respect to the output signal 2A of the load cell 9 as will be clear from FIG. 2, the change may be proximated as being linear with the rotations of the backup rolls 1 and 4.
Further, the rolling load variation due to the roll eccentricity in an interval T.sub.2 during which the backup rolls 1 and 4 are rotated by one revolution, respectively, is obtained by subtracting components shown by linear lines A-B in the same figure from the output signal of the load cell 9 during the same interval. This is performed as follow. That is, components shown by linear line A'--B' is subtracted firstly from the output signal of the load cell 9 and then a mean value of the result is subtracted from the result. A result of the last subtraction is the rolling load variation component due to the roll eccentricity in the interval.
In other words, the first control method comprises the steps of obtaining the rolling load variation due to the roll eccentricity during the rolling operation of the sheet material 5 and utilizing the variation as the roll eccentricity compensation signal for the rolling mill. Therefore, this method is advantageous in that it can respond to changes of rolling conditions due to such as wear, damage or expansion of the rolls 1 and 4 or exchange of them. However, since this method can not completely exclude influences of the rolling load variation due to the thickness variation of the sheet material to be rolled, there is a limit in control in this sense.
The second control method;
The second control method may be practicized by using an apparatus shown in FIG. 3. That is, the method comprises the steps of rotating the work rolls 2 and 3 without the sheet material 5, i.e., with the rolls 2 and 3 being in contact with each other under a load, obtaining rolling load variations due to the eccentricity of the backup rolls 1 and 4, respectively, from the rolling load variation obtained from the load cell 9, memorizing the variations thus obtained respectively and utilizing the memorized data as the roll eccentricity compensation signal during the rolling operation. The apparatus in the same figure further comprises another pulse generator 13 related to the lower backup roll 4 because it is also necessary to detect the rolling load variations due to the lower backup roll eccentricity. The second control method will be described in more detail on the way of description of the present invention.
In any way, since in the second control method, the rolling load variation value due to the roll eccentricity is obtained under the condition of direct contact of the work rolls 2 and 3, the value is not influenced by the thickness variation of the sheet material 5 to be rolled. However, the rolling conditions under which the rolling load variation value is detected differ from those under which the rolling mill is actually controlled according to the value detected and, therefore, there may be control errors. That is, the second control method can not respond to changes in shape of the backup rolls 1 and 4 due to wear, damage or expansion thereof with time.